1. Field of the Invention
This invention relates to vertical motor pumps, and, more particularly to a vertical motor pump suitable for use as an electric pump of a submerged type for pumping up a liquid of low viscosity, such as a cryogenic liquid.
2. Description of the Prior Art
Generally, a vertical motor pump, such as a vertical submerged pump for handling a liquid of low viscosity, such as water, a cryogenic liquid, etc., has main bearings in the form of ball bearings formed of a corrosion resistant metal.
The ball bearings of this type are constructed such that they are lubricated by a liquid of low viscosity, so that they could not be expected to have a prolonged service life under high load as is the case with ball bearings of an oil lubricated type. Thus, various measures have been taken to prolong their service life. A ball bearing usually comprises an inner race, an outer race and ball bearing elements, all formed of corrosion resistant metal. When a ball bearing is intended for use with equipment in which lubrication with a liquid of low viscosity is inevitable, the following lubrication techniques are used. A cage is formed of special resinous material provided by reinforcing expensive tetrafluoroethylene resin with glass fibers and mixing a self-lubricating agent, such as molybdenum disulfide (MoS.sub.2) powder with the resin. When the cage of this type is used, the MoS.sub.2 is transferred to the balls and adheres thereto as the balls are brought into contact with the cage during operation, to thereby achieve lubrication. However, even if this system is used, it would be difficult to prolong the service life of the ball bearings as one might wish. Thus, attempts have been made to adopt a load reducing system which is intended to reduce a load applied to the bearings to prolong their service life. A vertical motor pump provided with this load reducing system comprises a casing plate placed on an upper flange of an outer casing, and an inner casing located at the casing plate. The inner casing has secured to its interior a stator including a stator core and a stator coil, and a rotor including a rotor core and a rotor coil is arranged in predetermined spaced relation to the stator, to provide a motor section. Pump members, such as an impeller, are located in a position below the motor section. The rotor of the motor section and the impeller or a pump member are connected together by a rotary shaft for rotation therewith. The rotary shaft is journaled at its motor end by two ball bearings located one above the other and at its pump end by a radial bearing located at a lower end of the rotary shaft. A thrust load reducing system in the form of a rotary ram referred to hereinabove is mounted midway between the motor section and the pump section, to keep a pump thrust generated during pump operation from being directly applied to the ball bearings as a load.
Operation of the motor pump of the aforesaid construction of the prior art will be described. Energization of the stator starts the rotor so that the rotary shaft fitted to the rotor and the impeller attached to a lower portion of the rotary shaft begin to rotate. Rotation of the impeller causes a liquid on the suction side to flow into the impeller blades and has its pressure raised by the impeller arranged in a plurality of stages before flowing into a discharge passage extending behind the stator and reaching a channel from which it is transferred to an external conduit. In this process, lubrication of the bearings supporting the rotor is effected by the liquid having its pressure raised by the impeller. The lubrication of the ball bearings located one above the other on the motor side will first be described. A portion of the pressurized liquid flows into a gap in the rotary ram of the load reducing system from a chamber before the majority of the pressurized liquid flows into the discharge passage. At this time, an upwardly oriented thrust is produced in the rotary ram by the raised pressure of the liquid while a thrust is produced in the pump by the impeller back pressure. These two thrusts cancel each other out, thereby achieving the effect of reducing a load applied to the lower ball bearing. The liquid flowing out of the gap in the rotary ram is introduced into the lower ball bearing and flows into a gap between the stator and the rotor while lubricating the ball bearing, to effect cooling of portions heated by a current passed thereto. Then the liquid lubricates the ball bearing located at an upper end of the rotary shaft and gathers together in an overflow pipe before being returned to the suction side (in this case the liquid may flow through a heat exchanger before being returned to the suction side). Let us now study the behavior of the rotary ram during a transitional period of operation. When the pump is inoperative, it moves downwardly to a lowermost position due to the weight of the rotor. When the pump starts operation, the rotary ram is moved upwardly by the pressurized liquid and the rotary shaft fitted thereto begins to shift, and the upper and lower ball bearings also shift by following suit.
The load reducing system thus has the effect of causing the upper and lower ball bearings to quickly follow up the vertical movement of the rotary shaft to avoid occurrence of abnormal conditions in the ball bearings (inordinate wear or seizure caused by unbalanced movement or lopsided loading), so that it is possible to prolong the service life of the ball bearings. However, an increase in the capacity of pumps successively increases a load applied to the pumps. The load reducing system would be able to cope with an increase in the thrust applied to the pumps if its specifications were altered. However, if the equipment were high in head, the circumference of an inner chamber of an impeller housing might become unbalanced hydrodynamically, to thereby generate a force which would act as a radial thrust on the bearings. Production of this force is governed to a great extent by the degree of precision with which the impeller housing, flow regulating plate and impeller blades are fabricated, and the direction in which the force acts as a load is not constant. Thus, it is impossible to absorb the force by the load reducing system. The result of this is that the lower ball bearing most suceptible to a radial thrust would have its service life shortened.
Generally, calculation of the service life of a ball bearing is done by using equation (1) set forth below. To achieve a prolongation of the service life, it is considered effective either to minimize the value of the working load P or increase the capacity of the basic rated load C in equation (1). However, the capacity of the basic rated load C would be decided upon when decisions are made on the model and dimensions, and a reduction in the working load P would be the last available means for prolonging the service life. This last available means could not be used when it is impossible, as described hereinabove, to reduce the load. Thus it would be impossible to obtain a prolongation of the service life. EQU L=K.multidot.(C/P).sup.3 ( 1)
where:
L: service life; PA1 K: coefficient of lubrication PA1 C: basic rated load PA1 P: working load.